1. Field of the Invention
The present invention relates generally to a method of increasing the battery lifetime of a mobile station (MS) during a voice call in a mobile communication system, and in particular, to a method of supporting FCH (Fundamental Channel) gating for a voice call in a base station transceiver sub-system (BTS) and a base station controller (BSC).
2. Description of the Related Art
In general, a gating mode is a process of transmitting radio frames at a lower data rate than the previous data rate in the absence of transmission data in a mobile communication system. Reverse channel gating ensures an acceptable performance despite transmission of fewer frames than usual, thereby increasing the battery lifetime of an MS. Particularly, a voice service at a gating mode can maximize the profits of the gating.
An FCH, a pilot channel, and a dedicated control channel in a physical channel operate in a gating mode on the reverse link.
FIG. 1 illustrates a 3G IOS (3rd Generation Interoperability Specifications) reference model of a CDMA mobile communication system having an MSC (Mobile Switching Center), BS (Base Station)s, and a digital air interface between the BSs.
Referring to FIG. 1, an A1 interface carries signaling information between an MSC 20 and a BS 30, and A2 and A5 interfaces (dedicated to circuit data) carry user traffic. An A3 interface carries signaling information and user traffic between an SDU (Frame Selection/Distribution Unit) 34 of the source BS 30 and a target BS 40 in order to implement a soft/softer handoff between the BSs. An A7 interface carries signals between the target BS 40 and the source BS 30, for the soft/softer handoff. A forward link directed from the MSC 20 to the BS 30, a reverse link directed from the BS 30 to the MSC 20, and a link between the BS 30 and the BS 40 are wired communication links. The MSC 20 includes a call control and mobility management block 22 and a switch 24. The MSC 20 is connected to a data network (not shown) via an IWF (Inter-Working Function) 50. A8 and A9 interfaces carry signaling information and user traffic between each BS and a PCF (Packet Control Function) 60. A10 and A11 interfaces carry signaling information and user traffic between the PCF 60 and a PDSN (Packet Data Serving Node) 70.
FIG. 2A is a diagram illustrating a signal flow for implementing a hard handoff of an FCH in a conventional mobile communication system. Referring to FIG. 2A, if the signal strength of an MS is less than a threshold set in a network, the source BS determines to perform a hard handoff to at least one cell of the target BS 40. The source BS 30 transmits a list of corresponding hard handoff cells and a Handoff Required message to the MSC 20, activating a timer T7 in step 2a. In step 2b, the MSC 20 checks a hard handoff bit set in the Handoff Required message. If the hard handoff bit is set to 1 indicating a hard handoff, the MSC 20 transmits a Handoff Request message containing “TIA/EIA-95 Channel Identity”, for example, to the target BS 40. In the case of a hard handoff for asynchronous data or a fax service, the Handoff Request message includes a CIC (Circuit Identity Code) extended information element indicating the CIC between the SDU 34 and the IWF 50. Upon receipt of the Handoff Request message from the MSC 20, the target BS 40 connects a call to the MS by assigning radio resources set in the Handoff Request message to the MS and transmits null forward traffic channel frames to the MS in step 2c. 
In step 2d, the target BS 40 transmits a Handoff Request Acknowledge message to the MSC 20 and activates a timer T9 until the MS is identified on the assigned radio channel. The MSC 20 prepares for switching a call from the source BS 30 to the target BS 40, transmits a Handoff Command message to the source BS 30, and terminates the timer T7 in step 2e. The source BS 30 transmits one of a General Handoff Direction message, an Extended Handoff Direction message, and a Universal Handoff Direction message to the MS and activates a timer T8 in step 2f. If the MS is allowed to return to the source BS 30, a timer Twaitho (Timer WAIT Hand Off) is also activated.
In step 2g, the MS transmits an MS Ack Order message to the source MS 30 in response to the Handoff Direction message. The source BS 30 terminates the timer T8. If the Handoff Direction message is transmitted by fast repetition, the source BS 30 does not request acknowledgement from the MS. In this case, the timer T8 is not activated in step 2f. In step 2h, the source BS 30 transmits a Handoff Commenced message to the MSC 20 to notify that the MS is ready to switch to a channel of the target BS 40, and a timer T306 is activated until a Clear Command message is received from the MSC 20. If the timer Twaitho was activated, the source BS 30 waits to terminate the timer Twaitho before transmitting the Handoff Commenced message.
In step 2i, the MS transmits a reverse traffic channel frame or a traffic channel preamble to the target BS 40. The MS then transmits a Handoff Completion message to the target BS 40 in step 2j and the target BS 40 wirelessly transmits a BS Ack Order message for the Handoff Completion message to the MS in step 2k. In step 2l, the target BS 40 transmits a Handoff Complete message to the MSC 20 to notify that the hard handoff is successful for the MS, and terminates the timer T9.
In step 2m, the MSC 20 transmits a Clear Command message to the source BS 30 upon receipt of the Handoff Complete message from the target BS 40, and the source BS 30 terminates the timer T306. The MSC 20 activates a timer T315. In the case of a hard handoff for asynchronous data or a fax service, the MSC 20 releases all resources including an A5 connection from the source BS 30 by the Clear Command message. In step 2n, the source BS 30 transmits a Clear Complete message to the MSC 20 to notify that the release is successfully completed and the MSC 20 terminates the timer T315.
FIG. 2B is a diagram illustrating a signal flow for implementing a soft handoff of an FCH in the conventional mobile communication system. Referring to FIG. 2B, at a soft handoff the source BS 30 selects the target BS 40 that can offer radio resources to the MS and at least one of the cells of the target BS 40 in step 3a. The source BS 30 transmits an A7-Handoff Request message to the target BS 40 to request assignment of radio resources from the soft handoff cells and activates a timer Thoreq (Timer Hand Off REQuest). In step 3b, the target BS 40 determines soft handoff cells which can offer radio sources, transmits an A3-Connect message to the source BS 30 to establish new radio links for transmitting forward/reverse user data frames, or add a connection to an existing connection, and activates a timer Tconn3 (Timer CONNect 3). The source BS 30 transmits an A3-Connect Ack message to the target BS 40 in response for the A3-Connect message in step 3c. At the same time, the target BS 40 terminates the timer Tconn3. In step 3d, the source BS 30 transmits forward frames to the target BS 40. The target BS 40 transmits reverse idle frames to the source BS 30 for tuning in step 3e. 
In step 3f, the target BS 40 transmits forward frames to the MS on its assigned radio channel. The target BS 40 transmits an A7-Handoff Request Ack message to the source BS 30, indicating cells to which radio resources have been successfully assigned and the source BS 30 terminates the timer Thoreq in step 3g. After the source BS 30 is notified of successful reception of its forward frames and commencement of wireless transmission in the target BS 40, if the SDU 34 of the source BS 30 is well tuned to the target BS 40 and a traffic connection is established between the SDU 34 and the target BS 40, the target BS 40 transmits an A3-Traffic Channel Status message to the source BS 30 in step 3h. 
In step 3i, the source BS 30 transmits a Handoff Direction message to the MS to enable the MS to add the new cells to its active set. The MS transmits an MS Ack Order message to the source BS 30 in response to the Handoff Direction message in step 3j. In step 3k, the MS transmits a Handoff Completion message to the source BS 30, notifying that the Handoff Direction message has been successfully processed. The source BS 30 transmits a BS Ack Order message to the MS in response for the Handoff Completion message in step 3l. In step 3m, the source BS 30 can selectively transmit a Handoff Performed message to the MSC 20, indicating a soft handoff success.
The above handoff (soft handoff and hard handoff) procedures have the following problems on the side of a BS.
(1) There is no way of notifying a neighbor BS of the current reverse link gating mode in progress at a hard handoff of an FCH for a voice call. Therefore, the gating mode is not available to a new FCH assigned by a target BS after the hard handoff.
(2) There is no way of notifying a neighbor BS of the current reverse link gating mode in progress at a soft handoff of an FCH for a voice call. Therefore, the gating mode is not available to a new FCH assigned by a target BS after the soft handoff. An MS at the soft handoff receives FCH frames of source and target BSs from the BSC of the source BS. In comparison, it turns out that the FCH frame from the target BS is inferior in quality to the FCH frame from the source BS. Consequently, signal quality is deteriorated after the MS moves to the target BS and thus a call is disconnected.
(3) In the case where the MS is assigned an FCH by call origination or call termination, it can set a reverse FCH gating mode while the FCH is assigned. However, such an operation is not defined yet between a BTS (Base station Transceiver Sub-system) and a BSC. That is, the reverse FCH gating is not supported at present.